Method of making a semiconductor device using a low dielectric constant material

ABSTRACT

This invention provides a process for making a semiconductor device with reduced capacitance between adjacent conductors. This process can include applying a solution between conductors 24, and then gelling, surface modifying, and drying the solution to form an extremely porous dielectric layer 28. A non-porous dielectric layer 30 may be formed over porous layer 28, which may complete an interlayer dielectric. A novel process for creating the porous dielectric layer is disclosed, which can be completed at vacuum or ambient pressures, yet results in porosity, pore size, and shrinkage of the dielectric during drying comparable to that previously attainable only by drying gels at supercritical pressure.

FIELD OF THE INVENTION

This invention relates generally to the fabrication of dielectrics onsemiconductor devices, and more particularly to methods for reducingcapacitive coupling on a semiconductor device using electricalinsulators made of porous dielectric materials.

BACKGROUND OF THE INVENTION

Semiconductors are widely used in integrated circuits for electronicdevices such as computers and televisions. These integrated circuitstypically combine many transistors on a single crystal silicon chip toperform complex functions and store data. Semiconductor and electronicsmanufacturers, as well as end users, desire integrated circuits whichcan accomplish more in less time in a smaller package while consumingless power. However, many of these desires are in opposition to eachother. For instance, simply shrinking the feature size on a givencircuit from 0.5 microns to 0.25 microns can increase power consumptionby 30%. Likewise, doubling operational speed generally doubles powerconsumption. Miniaturization also generally results in increasedcapacitive coupling, or crosstalk, between conductors which carrysignals across the chip. This effect both limits achievable speed anddegrades the noise margin used to insure proper device operation.

One way to diminish power consumption and crosstalk effects is todecrease the dielectric constant of the insulator, or dielectric, whichseparates conductors. Probably the most common semiconductor dielectricis silicon dioxide, which has a dielectric constant of about 3.9. Incontrast, air (including partial vacuum) has a dielectric constant ofjust over 1.0. Consequently, many capacitance-reducing schemes have beendevised to at least partially replace solid dielectrics with air.

U.S. Pat. No. 4,987,101, issued to Kaanta et al., on Jan. 22, 1991,describes a method for fabricating gas (air) dielectrics, whichcomprises depositing a temporary layer of removable material betweensupports (such as conductors), covering this with a capping insulatorlayer, opening access holes in the cap, extracting the removablematerial through these access holes, then closing the access holes. Thismethod can be cumbersome, partially because it requires consideration ofaccess hole locations in the design rules and alignment error budgetduring circuit design, as well as requiring extra processing steps tocreate and then plug the holes. This method may also create large voidareas which have essentially no means of handling mechanical stress andheat dissipation.

U.S. Pat. No. 5,103,288, issued to Sakamoto, on Apr. 7, 1992, describesa multilayered wiring structure which decreases capacitance by employinga porous dielectric with 50% to 80% porosity (porosity is the percentageof a structure which is hollow) and pore sizes of roughly 5 nm to 50 nm.This structure is typically formed by depositing a mixture of an acidicoxide and a basic oxide, heat treating to precipitate the basic oxide,and then dissolving out the basic oxide. Dissolving all of the basicoxide out of such a structure may be problematic, because small pocketsof the basic oxide may not be reached by the leaching agent.Furthermore, several of the elements described for use in the basicoxides (including sodium and lithium) are generally consideredcontaminants in the semiconductor industry, and as such are usuallyavoided in a production environment. Creating only extremely small pores(less than 10 nm) may be difficult using this method, yet thisrequirement will exist as submicron processes continue to scale towardsa tenth of a micron and less.

Another method of forming porous dielectric films on semiconductorsubstrates (the term "substrate" is used loosely herein to include anylayers formed prior to the conductor/insulator level of interest) isdescribed in U.S. Pat. No. 4,652,467, issued to Brinker et al., on Mar.24, 1987. This patent teaches a sol-gel technique for depositing porousfilms with controlled porosity and pore size (diameter), wherein asolution is deposited on a substrate, gelled, and then cross-linked anddensified by removing the solvent through evaporation, thereby leaving aporous dielectric. This method has as a primary objective thedensification of the film, which teaches away from low dielectricconstant applications. Dielectrics formed by this method are typically15% to 50% porous, with a permanent film thickness reduction of at least20% during drying. The higher porosities (e.g. 40%-50%) can only beachieved at pore sizes which are generally too large for suchmicrocircuit applications. These materials are usually referred to asxerogels, although the final structure is not a gel, but an open-pored(the pores are generally interconnected, rather than being isolatedcells) porous structure of a solid material.

SUMMARY OF THE INVENTION

The present invention provides a method for forming highly porous,finely pored (pore diameter of less than 80 nm and preferably of 2 nm to25 nm), low dielectric constant (k less than 3.0 and preferably lessthan 2.0) dielectric films for use as semiconductor insulators.Surprisingly, the methods of this invention can provide an extremely lowdielectric constant insulation structure, formed from a wet gel withcontrolled shrinkage, without employing exotic production techniques orincurring disadvantages found in other low dielectric constant methods.

A previously unrecognized problem in the application of dried geldielectrics to microcircuits recognized herein is the shrinkagetypically observed during gel drying, which may cause mechanicalimperfections such as large voids, cracks, powdering, loose dielectricfragments, and stresses in surrounding structure, as well asdensification (and increased dielectric constant) of the dielectriclayer itself. Mechanical imperfections are particularly undesirable (andlikely) when the porous material is required to fill a high-aspect ratio(height greater than width) gap between adjacent conductors, such asthose commonly found on submicron integrated circuits, as shrinkage insuch gaps may pull the dielectric loose from the bottom and/or sides ofthe trench. The primary underlying cause of xerogel shrinkage duringdrying has now been recognized as resulting from capillary pressurecreated at the boundary between liquid and vapor solvent in the poresduring drying. The methods of this invention provide a novel solutionfor controlling densification and other shrinkage effects, which can beeasily applied to semiconductor fabrication, resulting in asubstantially undensified, highly porous rigid structure which can beformed even in high aspect ratio geometries.

Some of the other advantages possible with the present invention are:the processing can be done at atmospheric pressure, which not onlysimplifies processing but allows the construction of multiple porouslayers on the same device; the solvents can be removed essentiallycompletely from the porous film; the materials used in the process arenot harmful to semiconductor devices; the porous structure can be madehydrophobic (water repelling); high temperatures are not required at anystage in the application; the dried porous structure has adequatestructural strength to allow deposition of other layers on top of it;and, importantly, the pores formed in the dielectric can be made smallenough to allow this method to be used with device feature sizes in the0.5 to 0.1 micron range, or even smaller.

The present invention can provide a method for forming a porousdielectric film on a semiconductor device for the primary purpose ofdecreasing unwanted capacitive coupling between conductors on thesemiconductor device. The method can include providing a layercontaining at least two patterned conductors formed on a substrate anddepositing a thin film on the substrate from a non-gelled solution whichmay then be gelled on the substrate to form a wet, open-pored porous gel(in the wetted state, the pores of the gel are filled with liquid).Gelation is preferably accomplished by hydrolysis and condensation ofmetal alkoxides, gelling of particulate or colloidal metal oxides,gelation of organic precursors, or a combination of these approaches.The method can further comprise aging the wet gel for a predeterminedperiod of time under controlled temperature conditions. The method canfurther comprise performing a solvent exchange on the wet gel to removesubstantially all water from the gel structure. The method can furthercomprise reacting the wet gel with a surface modification agent. Thisreaction preferably causes the replacement of at least 15% (and morepreferably, at least 30%) of highly reactive groups (e.g. hydroxyl oralkoxyl groups) present on the internal pore surfaces with more stablesurface groups or ions (e.g. organic, fluorine, fluorocarbon), therebyat least partially preventing condensation reactions between neighboringgroups on the internal pore surfaces during drying, and therebycontrolling densification. The surface modification may alsosubstantially increase the pore fluid contact angle within the pores ofthe wet gel, thereby reducing capillary pressure duringnon-supercritical drying. The surface modification may also render theporous structure hydrophobic. The method can further comprise preferablydrying the gelled film at one or more sub-critical pressures (fromvacuum to near-critical) and more preferably, at atmospheric pressure,or alternately (but not preferably) drying the gelled film undersupercritical conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention, including various features and advantages thereof, canbe best understood by reference to the following drawings, wherein:

FIG. 1 shows a block diagram of the steps in a typical embodiment of theinvention;

FIGS. 2A-2D show cross-sectional illustrations of a solvent-filled pore,before and during solvent evaporation;

FIGS. 3A-3D show cross-sections of a portion of a semiconductor device,illustrating several steps in the application of an embodiment of theinvention to a typical device;

FIGS. 4A-4C show cross-sections of another semiconductor device,illustrating two separate applications of the present invention;

FIG. 5 shows a cross-section of another structure formed with themethods of the current invention, with a relatively thick porousdielectric and a relatively thin non-porous dielectric;

FIG. 6A-6H show cross-sections of yet another semiconductor device witha non-porous dielectric formed by two sublayers;

FIG. 7 shows a cross-section of a semiconductor device containing apassivation layer which isolates a porous dielectric layer from directcontact with the conductors; and

FIGS. 8A-8D show cross-sections of a semiconductor device withdielectric spacers affixed to the tops of conductors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Typical embodiments of the invention may be comprised of the steps shownin FIG. 1, although not all steps shown may be required in a givenembodiment. Furthermore, materials may be substituted in several of thesteps to achieve various effects, and processing parameters such astimes, temperatures, pressures, and relative concentrations ofingredients may be varied over broad ranges. In FIG. 1, variousprecursor solutions (some of which are described in detail in thespecific examples) may be mixed, and then applied to a substrate uponwhich a layer of patterned conductors has been formed. The method ofapplication may be, for example, a spin-on technique in a controlledatmosphere which limits solvent evaporation. The object of theapplication in at least one embodiment is to form a layer of theprecursor which will at least substantially fill the gaps betweenadjacent conductors. The precursor solution is allowed to gel on thesubstrate, a process which typically takes from 1 minute to 12 hours,depending on the solution and method of gelling. The wet gel can beallowed time to age, generally about a day (although it may be muchshorter), at one or more controlled temperatures. If the wet gelcontains water, one or more washing steps can be used to perform asolvent exchange on the gel, thereby removing the water but leaving thegel in a wet state. The solvent may be either a protic (e.g. ethanol) oran aprotic (e.g. acetone or hexane) solvent. The wet gel may then bereacted with a surface modification agent (the effects of the surfacemodification step will be explained below) by a method such as immersingthe structure in a mixture containing the surface modification agent anda solvent in which the modification agent is soluble. This solvent mustalso be miscible with the solvent already present in the wet gel.Another solvent exchange may be subsequently used to remove excesssurface modification agent from the structure. The solvent is allowed toevaporate out of the gel, leaving a porous dielectric structure. If thefilm is substantially undensified during drying, the dried gel exhibitsessentially the same structure as the wet gel (the dried film thicknessis substantially the same as the wet gel film thickness). The porousdielectric may finally be capped with a non-porous insulation layer, asdetailed in the specific examples.

Referring to FIG. 2A, a cross-section of a single pore 12 in a wet gelstructure 10 is shown, with a liquid pore fluid 14 filling pore 12. FIG.2B shows the same pore undergoing evaporation of the pore fluid. A phasechange (from liquid to vapor) is illustrated by the formation of ameniscus 18, which is shown as a crescent-shaped boundary between liquidpore fluid 14 and vapor 16 formed during evaporation. The meniscus is anindication of the surface tension of the pore fluid exerting an inward(usually, although some fluids can exert outward) pressure on the wallsof the pore. This capillary pressure P can be related to the pore fluidsurface tension T_(s), the contact angle q (the angle at which the fluidmeniscus contacts the surface of the pore), and the pore radius r, bythe equation ##EQU1## The difficulty in maintaining the integrity ofextremely small pores (small r) during drying is evident from thisequation, since every halving of radius r doubles the pressure on thepore walls. Unfortunately, a porous dielectric suitable for use betweenconductors should contain pores at least an order of magnitude smallerthan the interconductor gap (r approximately 10 nanometers for a 0.2micron gap, for example). Adjusting pore size upwards to relievecapillary pressure is therefore a limited option for microelectronicapplications. On the other hand, simply allowing pores to collapse fromthe capillary pressure results in excessive shrinkage, with thecorresponding densification of the dielectric defeating the primarypurpose of the method (reducing dielectric constant) as well aspreventing good surface adhesion.

To circumvent the capillary pressure problem in monolithic xerogelsynthesis, the aerogel technique has been developed. Generally, thisvariation of the xerogel technique removes a solvent from a wet gelunder supercritical pressure and temperature conditions. By removing thesolvent in the supercritical region, vaporization of the liquid solventdoes not take place; instead, the fluid undergoes a constant change indensity during the operation, changing from a compressed liquid to asuperheated vapor with no distinguishable state boundary. This techniqueavoids the capillary pressure problem entirely, since no state changeboundaries ever exist in the pores. Adapting the aerogel technique tosemiconductor fabrication appears to be problematic and expensive;typical solvent candidates have high critical pressures (e.g. ethanol,924 psi, carbon dioxide, 1071 psi) which make application difficult inmost circumstances. For instance, these pressures may tend to crushprevious layers or porous dielectric capped under atmospheric pressureor force the wet gel into the pores of previous porous dielectric layersleft uncapped, and may require containment of the wet gel at the edgesof the water to prevent the gel from being squeezed off the wafer beforethe gel can be dried. Nevertheless, a highly porous, finely poreddielectric structure may be formed by this process under someconditions, making this supercritical technique possibly useful in thepractice of the present invention.

As an alternative to this, the present invention includes a group ofnovel techniques which may be applied at a range of pressures fromvacuum to near-critical, with atmospheric pressure being preferable dueto ease of handling and compatibility with previous porous layers. Onesimilarity in these techniques is that a surface modification step isperformed on the wet gel, replacing a substantial number of themolecules on the pore walls with those of another species. This surfacemodification typically replaces reactive surface groups such ashydroxyls and alkoxyls with more stable surface groups such as methylgroups, thereby controlling undesirable condensation reactions (andshrinkage effects) during gel drying. FIG. 2C shows a cross-section of apore after the surface modification step; portions of gel 10 which areon the surface of pore 12 (labeled as region 20) now contain a differentspecies. It has been discovered that by controlling the percentage ofreactive surface groups replaced during the surface modification, thefinal shrinkage may be adjusted from the large shinkage typical of anunmodified xerogel (with uncontrolled shrinkage) to a shrinkage of onlya few percent, heretofore only achievable with an aerogel technique.Typically, approximately 30% of the reactive surface groups must bereplaced to substantially alleviate densification. Furthermore, thereplacement surface species may be chosen because of its wettingproperties in combination with specific pore fluids; thus in FIG. 2D,meniscus 18 is significantly flatter than that of FIG. 2B, resulting ina pore fluid contact angle closer to 90 degrees. As the fluid contactangle approaches 90 degrees, the cosine of the contact angle q goes to0, and the capillary pressure P of Equation 1 is reduced proportionally.It is believed that the surface modification prevents surfacecondensation reactions, and may also reduce capillary pressure bychanging pore fluid contact angle, thereby allowing pores in the surfacemodified gel to better survive drying. This novel technique can producea dielectric layer, at atmospheric pressure, with average pore diameter,porosity, and overall shrinkage resembling those ofsupercritically-dried aerogels.

An additional benefit of the surface modification can be hydrophobicity.It has been found that, for example, replacing only 15% of the reactivesurface groups with methyl groups may be sufficient to cause thestructure to be hydrophobic. This is an important feature for anymaterial used in semiconductor processing, but particularly so forporous materials. If the porous surfaces are left hydrophilic(water-wanting), the structure is in many ways analagous to a commonhousehold sponge, which may hold many times its weight in water.However, the extremely small pore sizes allow a hydrophilic porousdielectric to rapidly gather water out of the surrounding air, theprevention of which would be an added difficulty during devicefabrication. By making the pores hydrophobic before the gel is dried,these types of difficulties may be avoided.

In accordance with the present invention, FIGS. 3A-3D showcross-sections of a semiconductor device at various stages duringfabrication. During the description of the embodiments, use of the wordwafer will imply a wafer as used in conventional semiconductorprocessing, with at least the illustrated semiconductor deviceincorporated therein. In FIG. 3A, three patterned conductors 24 (e.g. ofaluminum alloyed with a small amount of copper) are shown formed on aninsulating layer 22, which may contain vias or through holes (not shown)for providing electrical contact between conductors 24 and lower layersof the device. A precursor solution 26 is shown disposed betweenconductors 24, after application to the wafer, for example, by a spin-ontechnique. The precursor may be prepared, for example, by the following2-step process. First, TEOS stock, a mixture of tetraethylorthosilicate(TEOS), ethanol, water, and HCl, in the approximate molar ratio1:3:1:0.0007, is prepared by stirring these ingredients under constantreflux at 60 degrees C. for 1.5 hours. Secondly, 0.05 M ammoniumhydroxide is added to the TEOS stock, 0.1 ml for each ml of TEOS stock.Since the addition of the ammonium hydroxide to the stock greatlyincreases gelation rate, the solution must be quickly applied to thewafer (it may be possible to switch the order of these two steps). Afterthe solution is applied to the wafer, care should be taken to insurethat the thin film does not dry prematurely; preferably, the wafercontaining the solution/gel remains immersed either in liquid or in asaturated atmosphere at all times prior to the drying stage. Gelationand aging may preferably be accomplished by letting the device sit in asaturated ethanol atmosphere for approximately 24 hours at about 37degrees C. Next, the water may be removed from the wet gel, preferablyby immersing the wafer in pure ethanol. The surface modification stepmay then be performed, preferably by immersing the wafer in a hexanesolution containing about 10% by volume trimethylchlorosilane (TMCS).After a brief reaction time, the unreacted surface modification compoundis usually removed by immersing the wafer in an aprotic solvent (e.g.acetone, hexane) and allowing excess solvent to drain. After thissolvent exchange, solvent is finally allowed to evaporate from the wetgel 26. This may produce a structure similar to that of FIG. 3B, whichillustrates the dried gel now forming a porous dielectric layer 28, andalso illustrates the few percent shrinkage typical of this method (thedried porous film thickness is only slightly less than the wet gelthickness). One advantage of this and similar embodiments is that thesurface-modified porous dielectric layer is hydrophobic, whereas anotherwise similar supercritically-dried aerogel (without surfacemodification) tends to be hydrophilic unless subsequently treated.

It is preferable to, as shown in FIG. 3C, cap porous layer 28 with asubstantially non-porous dielectric layer 30 to seal the open-poredstructure, mechanically reinforce the device, and to provide anon-porous layer for via etching and constructing furthermetal/dielectric layers. This layer may be comprised of silicon dioxide,silicon nitride, a composite layer having silicon dioxide and siliconnitride sublayers, silicon oxynitride, an organic insulator, or similarmaterials, applied by a method such as chemical vapor deposition (CVD)or as a spin-on glass (SOG). FIG. 3D shows a via etched throughnon-porous layer 30 and filled with a conducting material to provide ametal-filled via 32, thereby providing a means for electrical connectionbetween a conductor 24 and a second layer of patterned conductors 34,one of which is shown. The non-porous layer in this embodiment forms themajority of the interlayer dielectric. Although the solid dielectric mayprovide little or no reduction in layer-to-layer capacitance, excellentinterlayer mechanical properties are maintained. This is preferred,because it achieves low intralayer capacitance and, at the same time,generally retains mechanical properties of a completely solidintra/interlayer dielectric. This recognizes that intralayer capacitancereduction is much more important than interlayer capacitance reduction.

FIGS. 4A-4C show a second embodiment with a different dielectricconfiguration. FIG. 4A shows a structure similar to that of FIG. 3C,with the one exception being that non-porous dielectric layer 30 is toothin to form the interlayer dielectric. Referring to FIG. 4B, a secondporous dielectric layer 36 is created, for example, by coatingnon-porous dielectric layer 30 with a non-gelled precursor solution andrepeating the steps of FIG. 1. A cap layer 38 may be deposited oversecond porous layer 36, as shown in FIG. 4C. Cap layer 38 may be formed,for instance, using similar materials and processes as those used tocreate non-porous layer 30. This embodiment can provide a substantiallylower interlayer dielectric constant than the previous embodiment,possibly at the expense of some structural strength. However, thenon-porous and cap layers can help control via formation, and the caplayer can provide a solid foundation for additional conducting layers.

FIG. 5 illustrates an embodiment with only one porous and one non-porousdielectric layer, but with the intralayer and most of the interlayerdielectric generally formed by the porous layer. Porous dielectric layer28 is preferably formed by increasing the deposited depth of the coatingsolution to completely cover the conductors to about the depth (measuredfrom substrate 22) required to form the interlayer dielectric. Thisprocess may require depositing and gelling solution several times tobuild the required insulator thickness. Porous dielectric layer 28 maythen be dried in accordance with one of the methods of the invention. Anon-porous layer 30 may be applied over porous layer 28, for instance,using similar materials and processes as those used to form non-porouslayers in the previous embodiments.

FIGS. 6A-6F show cross-sections of a device construction useful forporous intralayer dielectrics. FIG. 6A again shows patterned conductors24 on a substrate 22. By a method such as those disclosed above, forexample, a porous dielectric layer 28 is constructed to fill gapsbetween and cover conductors 24, with the dried structure possiblyresembling FIG. 6B. FIG. 6C shows the structure after removal of a topportion of porous layer 28 to preferably expose the tops of conductors24. The material removal may be accomplished, for example, by acontrolled chemical etch, such as HF plasma etching, with concentrationsand etch times strongly dependent on the dielectric porosity.Alternately, the material removal may be done with a mechanicalpolisher, using, for example, an aqueous colloidal suspension of silica.This recognizes that it may be easier (and therefore preferable) todeposit a thicker porous layer and etch it back than to more preciselydeposit the porous layer only in the gaps between conductors. FIG. 6Dshows a step of depositing, preferably by a chemical vapor deposition(CVD) technique, a conformal sublayer 56, of silicon dioxide forexample, directly over the porous dielectric layer 28 and the conductors24. A dry-processed CVD layer, which would primarily deposit near thetop of the porous layer, may be preferable to spin-on glass (SOG), whichmay contain solvents capable of wetting the pores in porous layer 28.However, CVD is not particularly planarizing, and is a relatively slowmethod for forming a thick dielectric. FIG. 6E illustrates how anon-porous dielectric 30 may be applied over conformal sublayer 56, forexample as an SOG oxide, to quickly complete a planarized interlayerdielectric.

FIG. 6F shows the structure after deposition and patterning of aphotoresist mask 50. This prepares the water for the etch of via 52through layers 30 and 56, as shown in FIG. 6G. An advantage of thisembodiment is that via 52 does not pass through porous dielectric 28,which may be a difficult material to pattern precisely. Finally, FIG. 6Hshows a metal-filled via 32 and one of a second layer of patternedconductors 34, electrically connected by metal-filled via 32 to one ofpatterned conductors 24. This embodiment of the invention can provideexcellent intralayer capacitance reduction, a good mechanical bondbetween porous and non-porous dielectrics, a straightforwardconstruction technique with largely conventional via formation, and aplanarized, non-porous interlayer dielectric with good mechanical andheat transfer characteristics.

FIG. 7 is included to illustrate an embodiment wherein porous dielectriclayer 28 is isolated from conductors 24 by a relatively thin conformalpassivation layer 54, which may be formed, for example, of a CVD silicondioxide. This layer may be advantageous in several embodiments. In anembodiment such as that of FIG. 6, layer 54 may be removed from the topsof conductors 24 during etchback of porous dielectric 28.

FIGS. 8A-SD illustrate an additional embodiment which includesdielectric spacers. In FIG. 8A, conductors 24 are patterned withdielectric spacers 58 on top of them. The spacers are preferably formedof the same material used in non-porous layer 30 (shown on FIG. 8D).This may be accomplished by depositing a conducting layer, overlayingthis with a dielectric layer of a material such as silicon dioxide, andpatterning both with one mask. In FIG. 8B, a porous dielectric layer 28has been formed to preferably cover spacers 58, as shown. FIG. 8C showsthe device after a top portion of porous dielectric 28 has been removed.This step preferably exposes the tops of the spacers, and, as FIG. 8Cillustrates, in practice a top portion of spacers 58 will probably beremoved as well. Finally, FIG. 8D shows the device after non-porousdielectric 30 has been deposited over the structure to complete theinterlayer dielectric. An advantage of this embodiment is that theaddition of the spacers allows the removal of a top portion of theporous dielectric, without the possibility of removing a portion of theconductors. This structure may also result in lower crosstalk, ascompared to the embodiment of FIG. 6.

The following table provides an overview of some embodimentscross-referenced to the drawings.

    ______________________________________                                        Drawing                                                                              Preferred or Generic   Other Alternate                                 Element                                                                              Specific Examples                                                                          Term      Examples                                        ______________________________________                                        22     Previous     Substrate Previously-formed                                      interlayer             layers of a semicon-                                   dielectric             ductor device                                   24,34  AlCu alloy   Conductors                                                                              Al, Cu, Mo, W, Ti,                                     and/or refractory      and alloys of these                                    metal                  Polysilicon, silicides,                                                       nitrides, carbides                              26     TEOS stock   Precursor Solution of particulate                                             solution  or colloidal silicon,                                                         germanium, titanium,                                                          aluminum silicate                                                             ratioed TEOS/                                                                 MTEOS (methyl-                                                                triethoxysilane) stock,                                                       ratioed TEOS/                                                                 BTMSE (1,2-Bis                                                                (trimethoxysilyl)                                                             ethane) stock                                   28,36  Surface-modified                                                                           Porous    Supercritically-dried                                  dried gel    dielectric                                                                              aerogel, other fine-                                                layer     pored porous                                                                  dielectrics                                     30,38  Silicon dioxide                                                                            Non-porous                                                                              Other oxides, B or                                                  dielectric                                                                              P-doped SiO.sub.2, silicon                                          layer     nitride, silicon                                                              oxynitride Parylene,                                                          polyimides, organic-                                                          containing oxide                                32     AlCu alloy   Metal-filled                                                                            Same as conductors                                     and/or refractory                                                                          via       above                                                  metal                                                                  50                  Photoresist                                               54     Silicon dioxide                                                                            Passivation                                                                             Silicon nitride, silicon                                            layer     oxynitride                                      56     Silicon dioxide                                                                            Conformal Silicon nitride, silicon                                            sublayer  oxynitride, organic-                                                          containing oxide                                58     Silicon dioxide                                                                            Dielectric                                                                              Same as non-porous                                                  spacers   dielectric layer                                ______________________________________                                    

The invention is not to be construed as limited to the particularexamples described herein, as these are to be regarded as illustrative,rather than restrictive. The invention is intended to cover allprocesses and structures which do not depart from the spirit and scopeof the invention. For example, one skilled in the an could apply one ofthe many other published methods of initially forming a wet gel from anappropriate precursor to this invention. Alternately, one couldsubstitute organics for a portion of the silica while, for example,still having a material which was principally silica (less than 50 atompercent of the silicon being replaced). Properties of some of thespecific examples may be combined without deviating from the nature ofthe invention.

What is claimed is:
 1. A method of forming a porous dielectric on asemiconductor device comprising:(a) providing a first conductor and ahorizontally adjacent second conductor, formed on a substrate wherein agap is formed between said first and said second conductors; (b)providing a solution capable of forming a wet gel; (c) coating saidsubstrate with said solution such that the gap between said first andsecond conductors is filled substantially with said solution; (d)gelling said solution to form a wet gel on said substrate, said wet gelcontaining pores arranged in an open-pored structure; and (e) dryingsaid wet gel to form a substantially undensified porous dielectriclayer, said porous dielectric having a dielectric constant less than 3.0and a pore diameter of less than 80 nm, whereby the capacitive couplingbetween conductors on the same level is substantially reduced comparedto a solid silicon dioxide dielectric.
 2. The method of claim 1, furthercomprising after said gelling step, washing said wet gel with a solventto substantially remove any water contained in said wet gel.
 3. Themethod of claim 2, further comprising, after said washing step, reactingsaid wet gel with a surface modification agent to replace at least 15%of reactive groups on the surface of said pores with substantiallystable surface groups, whereby unwanted condensation reactions and thedensification of said wet gel during said drying step are controlled. 4.The method of claim 3, wherein said reactive groups include hydroxylgroups.
 5. The method of claim 3, wherein said stable surface groups areorganic.
 6. The method of claim 3, wherein said drying step occurs atone or more subcritical pressures.
 7. The method of claim 3, whereinafter said reacting step, unreacted portions of said surfacemodification agent are removed from said wet gel.
 8. The method of claim1, wherein said gelling step is accomplished by methods selected fromthe group consisting of: hydrolyzing and condensing metal alkoxides,gelling of particulate or colloidal metal oxide, and gelling of organicprecursors, or a combination thereof.
 9. The method of claim 1, whereinsaid gelling step further comprises aging said wet gel for a period oftime at one or more temperatures lower than the boiling point of asolvent contained in said wet gel.
 10. The method of claim 1, whereinsaid pores in said porous dielectric have diameters in the approximaterange of 2 nm to 25 nm.
 11. The method of claim 1, wherein said porousdielectric is comprised principally of silicon dioxide.
 12. A method offorming a porous dielectric on a semiconductor device comprising:(a)providing a first conductor and a horizontally adjacent secondconductor, formed on a substrate wherein a gap is formed between saidfirst and said second conductors; (b) providing a solution capable offorming a wet gel; (c) coating said substrate with said solution suchthat the gap between said first and second conductors is filledsubstantially with said solution; (d) gelling said solution to form awet gel on said substrate, said wet gel containing pores arranged in anopen-pored structure: (e) replacing at least 15% of reactive groups onthe surface of said pores with substantially stable surface groups; and(f) drying said wet gel to form a porous dielectric layer, at one ormore sub-critical pressures, said porous dielectric having a dielectricconstant less than 3.0 and a pore diameter of less than 80 nm, wherebythe capacitive coupling between conductors on the same level issubstantially reduced compared to a solid silicon dioxide dielectric,and whereby unwanted condensation reactions and densification of saidwet gel during said drying step are controlled.
 13. The method of claim12, wherein said replacing step comprises:(a) washing said wet gel witha solvent to remove substantially any water contained in said wet gel;and (b) reacting said wet gel with a surface modification agent.
 14. Themethod of claim 12, wherein said stable surface groups are organic. 15.The method of claim 12, wherein said drying step is conducted at ambientpressure.
 16. The method of claim 12, wherein said pores in said porousdielectric have diameters in the approximate range of 2 nm to 25 nm. 17.A method of forming a porous dielectric on a semiconductor devicecomprising:(a) providing a semiconductor substrate; (b) preparing asolution capable of forming a wet gel; (c) coating said substrate with afilm of said solution, said film having an average thickness between 0.1microns and 2 microns; (d) gelling said film to form a wet gel on saidsubstrate, said wet gel containing pores arranged in an open-poredstructure; (e) performing a solvent exchange on said wet gel to removesubstantially all water contained in said gel; (f) reacting said wet gelwith a surface modification agent to replace at least 15% of reactivegroups on the surface of said pores with substantially stable surfacegroups; and (g) evaporating said solvent from said wet gel atapproximately atmospheric pressure to from a porous dielectric layer,whereby unwanted condensation reactions and the shrinkage of said wetgel during said drying step are controlled.
 18. The method of claim 17,wherein after step (f), a second solvent exchange is performed to removesubstantially all unreacted surface modification agent from said poresof said gel.
 19. The method of claim 17, wherein pores in said porousdielectric have diameters in the approximate range of 2 nm to 25 nm. 20.The method of claim 17, wherein said gelling step further comprisesaging said wet gel at one or more elevated temperatures for a timeperiod.